Methyl orange changes from red
Methyl orange changes from red at pH 3.1 to yellow at pH 4.4.Would methyl orange be a suitable indicator for the titration ofpropanoic acid (pKa=4.87, Ka=1.34×10^-5) with 0.01 M NaOH? Explain.Phenolphthalein changes from colorless at pH 8.0 to pink at pH 9.6.Would phenolphthalein be a suitable indicator for the titration ofpropanoic acid with 0.01 M NaOH? Explain.
Answer:
Indicator- is a dye(substance) which changes colour over a shortpH range.
an indicator is usually weak acid or weak base for acid-basetitration.the ccolour change occurs when a proton transferred toform a conjugate base or a conjugate acids of a differentcolour
Methyl orange (MeOH) is represents as
MeOH 
Me+ + OH–
(Yellow in base) (Red in acid)
When a strong acid is added,equilibrium of MeOH shifts towardsthe right and the colour change Red is detected.
Phenolphthalein (PhH)
PhH Ph+ + H+
(Colourless) (pink in base)
PhH is a colourless in acid solution and pink in basic solution.When a strong base such as NaOH is added it displaces theequilibrium towards right direction and there is a colurchange.
As in given question titration of propanoic acid(weak acid asorganic acid mostly weak) and NaOH(strong base),the pH range nearthe end point is 8-10.since the pH lies on alkalineside.phenolphthalein (pH range 8-9.6 or 9.8) and thymolblue(pH range 8.6-9.6)are suitable indicators for suchtitrations.methyl orange (pH range 3.1-4.4)cannot be used for suchtitrations.also propanoic acid is weak acid does notfurnish sufficient number of H+ ions.these H+ions is not sufficient to combine with sufficient number ofOH– ions of methyl orange to form water and to shift theequlibrium in favour of red colouration Me+ ions.
MeOH Me+ + OH–
CH3CH2COOH CH3CH2COO– + H +
CH3CH2COO– +Me+CH3CH2COOMe
H+ + OH– H2O
However,the salt of CH3CH2COOMe does notfurnish sufficient red ions because of hydrolysis
CH3CH2COOMe + H2O CH3CH2COOH + MeOH
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